There is considerable demand for short-wavelength laser sources such as green, blue and UV lasers. One known approach to create such a light source is to utilize red or infra-red laser diodes, which are widely available in a variety of configurations. These diodes, in combination with nonlinear elements made of optically nonlinear materials, can produce short-wavelength frequency-doubled radiation by means of second harmonic generation (SHG) in the nonlinear element.
A laser source for frequency doubling requires (a) high power, (b) stable, narrow-line operation, (c) simple, low-cost optics and assembly, and, importantly (d) some means of fine-tuning the spectrum to match it to a doubling material to optimize efficiency. This invention particularly relates to a method and circuit to minimize noise in the frequency doubled output signal.
A number of prior art designs for frequency doubling of laser diode emission have been disclosed. For example, U.S. Pat. No. 5,384,797, in the names of Welch et al., describes a monolithic multi-wavelength laser diode array having output light that can be coupled into a ferroelectric frequency doubler integrally formed on an array substrate. U.S. Pat. No. 5,644,584, in the names of Nam et al. describes a tunable blue laser diode having a distributed Bragg reflector (DBR) or distributed feedback (DFB) tunable diode laser coupled to a quasi-phase-matched waveguide of optically nonlinear material. U.S. Pat. No. 6,370,168 to Spinelli describes an intracavity frequency-converted optically-pumped semiconductor laser based on surface-emitting gain-structure surmounting a Bragg mirror, and an external concaved mirror. U.S. Pat. No. 6,393,038 to Raymond et al. describes a frequency-doubled vertical-external-cavity surface-emitting laser; and, U.S. Pat. No. 6,438,153 to Caprara et al. describes an intracavity-converted optically-pumped semiconductor laser. Although these aforementioned inventions appear to perform their intended function, many provide solutions wherein power and frequency stabilization requirements are met through the use of complex laser structures or complex nonlinear element arrangements. Furthermore, since complex laser designs usually lead to somewhat reduced power and increase in noise, these prior art solutions either use an intra-cavity nonlinear doubling arrangement to benefit from the intra-cavity resonance power enhancement, at the expense of yet more complex cavity control, or use single-pass doubling with relatively low output powers. U.S. Pat. No. 7,242,700 to Wang discloses a circuit for stabilizing a laser frequency doubled output signal. Although this circuit to some degree achieves its goal, a more reliable solution having less noise in the output laser frequency doubled signal is being sought after.
In an attempt to overcome some of the limitations of the prior art, U.S. Pat. No. 7,177,340 to Lang et al. assigned to JDS Uniphase Corporation describes an extended cavity laser device with bulk transmission grating. The system of Lang et al. uses a semi-conductor, high power, lasers of simple cavity design, such as edge emitting 980 nm laser diodes commonly used to pump erbium-doped fiber amplifiers, in an external cavity arrangement with frequency stabilization provided by an external frequency selective reflector. This patent attempts to provide a simple Littrow-type external cavity configuration for a diode laser, which substantially maximizes output power and enables frequency tuning without angular tuning of the output beam. An interesting aspect of U.S. Pat. No. 7,177,340 is that spectral noise of the laser diode radiation is substantially mitigated by providing an additional electric dither current to the laser diode to cause frequency modulation of the laser diode radiation with amplitude exceeding mode spacing. Such dithering results in a continuous scrambling of the laser light to increase stability of time-averaged spectrum of the laser diode radiation and therefore to stabilize a time-averaged power of the frequency-doubled radiation.
SHG is routinely used to double the frequency and halve the wavelength of near-IR fundamental radiation having a wavelength near 1000 nanometers (nm) to produce visible light having a wavelength near 500 nm. In this context, SHG commonly involves propagating an IR output beam from a diode-pumped solid-state (DPSS) laser resonator through an appropriate optically nonlinear crystal, for example a crystal of lithium triborate (LBO). When such a crystal is properly tuned by establishing an appropriate orientation and temperature, visible light is generated and exits the crystal, usually accompanied by some residual fundamental-wavelength light. The efficiency of converting power from the fundamental wavelength to the desired frequency-converted wavelength (conversion efficiency) is defined by the ratio of the net power transferred to the frequency-converted output divided by the power contained in the fundamental-wavelength source beam. IR-to-visible conversion efficiencies exceeding 50% in LBO are readily demonstrated.
As noted above, optimization of a frequency converter generally involves establishing an appropriate wavelength-dependent crystal orientation and operating temperature. Operating parameters that optimize conversion efficiency for a particular frequency converter may be determined during a preliminary characterization or calibration phase of system operation. Optimal operating parameters might be determined once and thereafter be left unchanged in anticipation of retaining the demonstrated performance without subsequent intervention by a system operator.
However, such an optimistic approach typically encounters problems. Over time, the frequency-converted output power of a DPSS laser, even given constant input power, tends to degrade as optical components age and accumulate damage. In addition, as the laser system power level or operating duty factor change, re-tuning of the frequency converter often becomes necessary and some method of monitoring conversion efficiency becomes advisable.
One method to compensate for deteriorating frequency-converted output power involves monitoring the frequency-converted output power level and increasing pump power as needed to boost the output while also monitoring the fundamental power level to allow determination of the conversion efficiency. Adjustments may then be made to the frequency converter to maintain or recover the desired efficiency. This approach does not always give satisfactory results.
The time-averaged power and perhaps even the position of the frequency-converted output may change so much over time that no reasonable adjustment of the diode drive current alone can recover the desired operating condition. Further, when fundamental and frequency-converted beams are sampled and detected separately, components exposed to the different beams may degrade at different rates. Such differential aging may bias the assessment of conversion efficiency. In addition, verifying that peak conversion efficiency is being maintained, when the output power may be slowly varying due to changes in the laser resonator, necessitates detuning the frequency converter away from an optimal condition, checking for a corresponding roll-off in performance, then re-tuning back to the optimal value.
In many instances, it is desirous to provide a frequency doubling laser circuit that maximizes output power regardless of the noise generated. However in other particular instances, where output power is of less of a concern, it is desired to minimize the noise present in the frequency doubled signal. Although maximizing output power and lessening noise typically occur to some degree together, they most often do not track perfectly with one another. For example achieving the lowest possible noise in the output signal does not ensure that maximum power will be attained. Furthermore, achieving maximum power output from the frequency doubling crystal does not ensure that the lowest noise will be achieved.
This invention is primarily concerned with the lessening of noise in the frequency doubled output signal and provides a novel way in which to achieve this as the laser and frequency doubling crystal physically change by aging over time.
It is an object of this invention to provide a method and circuit for frequency converting an optical signal to a shorter wavelength while minimizing noise in the output signal.
It is an object of this invention to continuously dither the temperature of a frequency doubling crystal and search for an optimized crystal temperature by varying the temperature in predetermined steps based on input from an RMS noise detector and current monitor.
It is an object of the invention to optimize the crystal temperature to minimize noise as the laser ages, as current driving the laser changes, as ambient temperature changes, and as stress and release of stress occurs within the crystal.